As materials for optical elements including lenses, glass materials and plastic materials are conventionally used. There are various types of glass materials, and they offer a variety of optical properties, which makes the optical design of optical elements easy. Further, glass materials are inorganic, and therefore it is possible to obtain optical elements having high reliability. In addition, by grinding glass materials, it is possible to obtain optical elements having a high degree of accuracy.
However, in order to obtain glass optical elements having an aspherical surface other than a flat or spherical surface, it is necessary to use a special grinding machine or to mold a glass material, which can be molded at low temperature, with an expensive molding die having high heat resistance (e.g., a ceramic molding die). Therefore, the production cost of such a glass optical element is high and the resulting glass optical element is very expensive.
On the other hand, optical elements using synthetic resins (plastics) can be produced at low cost by injection molding or casting. However, such optical elements have a problem in that they have low heat resistance, high thermal expansion, narrow range of choices in their optical properties such as refractive index, and low reliability.
In order to solve the above problems, there are proposed hybrid optical elements intended to have desired properties by laminating a resin layer on a glass substrate. For example, Japanese Patent Application Laid-open No. Sho 54-6006 discloses a low-pass filter obtained by forming an organic polymer layer on a flat glass substrate. Japanese Patent Application Laid-open Nos. Sho 52-25651 and Hei 6-222201 disclose aspherical lenses obtained by forming an aspherical resin layer on a glass lens substrate, that is, so-called hybrid aspherical lenses.
In recent years, such hybrid optical elements have come into use in various fields, and therefore reliability required of these hybrid optical elements is becoming increasingly higher. For example, there is a case where hybrid optical elements need to have durability to withstand exposure to a high temperature of 85° C. and a high humidity of 85% for 500 to 1,000 hours.
In order to achieve such durability, a hybrid optical element is conventionally formed by coating a silane coupling agent diluted with a solvent on a substrate and then forming a resin layer on the silane coupling agent coating to improve adhesion between the substrate and the resin layer. However, such a laminated optical element involves a problem that it is difficult for the optical resin layer to keep good adhesion to the substrate under severe conditions and therefore the separation of the optical resin layer is likely to occur.
The problem of separation of the optical resin layer can be solved by increasing the silane coupling agent content of the silane coupling agent coating, but this method causes another problem that the surface of the silane coupling agent coating becomes clouded or the silane coupling agent cannot be coated on the substrate evenly, that is, uneven coating occurs.
Japanese Patent Application Laid-open No. Hei 5-100104 discloses a hybrid optical element having a dielectric multilayer film comprising SiO2/ZrO2/SiO2 layers, the dielectric multilayer film being formed on a high refractive index glass substrate, whose high refractive index oxide content is high and a silica (SiO2) content is low, to prevent reflection caused by the difference in refractive index between the high refractive index glass substrate (refractive index: about 1.8 to 2.0) and an optical resin layer (refractive index: about 1.5).
Usually, as described above, adhesion between a glass substrate and an optical resin layer is improved by coating a silane coupling agent on the glass substrate because the silane coupling agent has the effect of improving the adhesion of the optical resin layer to silica. However, in a case where a high refractive index glass whose silica (SiO2) content is low is used as a glass substrate, adhesion between the glass substrate and the optical resin layer cannot be improved by the silane coupling agent. Although, as described above, the hybrid optical element disclosed in Japanese Patent Application Laid-open No. Hei 5-100104 uses a high refractive index glass as a glass substrate, the optical resin layer has good adhesion to the glass substrate because an SiO2 layer is provided as the uppermost layer of the dielectric multilayer film. However, the dielectric multilayer film is formed by vacuum evaporation, ion plating, or sputtering, which makes it difficult to produce such an optical element at low cost in a short time.